This invention relates to a method and system for separating into its constituent parts liquid mixtures of oil, gas and water. Many heater-treater systems have heretofore been proposed. The present invention is directed generally to a type of heater such as disclosed in U.S. Pat. No. 3,432,991 wherein the gas separating chamber and an emulsion treating chamber are formed within a horizontal vessel. The gas separating chamber has a heat exchange structure to accelerate gas separation and the emulsion treating chamber utilizes high voltage electrostatic fields for coalescing water particles and causing them to drop out of the stream within the emulsion treating chamber.